Is it possible to use a 11.1v 3s battery in a JR transmitter? Is there problems with doing this?
Do people do this?

The answers are: Yes, Maybe, and Yes.

To answer your last question first, I use a 3S2P, 2400 mAh Li-ion pack in my Spektrum DX6 (which is basically a JR 6102 with a 2.4GHz RF deck installed). This battery typically gives me two to four weeks of flying on a single charge, and I fly several times a week. And I built the pack from six 1200 mAh Li-ion cells from All Electronics, which cost a whopping two dollars each. I have been using this setup for months, with no problems of any sort...but I took precautions to drop the battery voltage a little, just for safety. Read on...

As to potential problems, the stock pack gets up to maybe 11.2 volts hot off the charger. A freshly charged 3S lipo pack puts out 12.6 volts for a short time, usually quickly dropping to below 12 volts. This slight increase over stock should not hurt any well designed Tx, but if you want to be conservative, there is a very simple fix: simply insert a silicon diode between the lithium battery and the transmitter circuit board.

The diode will drop about 0.7 volts, so the transmitter will "see" about 11.9 volts when a freshly charged 3S lipo is installed; this will usually very quickly drop to about 11.3 volts, which should be quite safe as it is very close to the stock 8-cell NiMH or NiCd voltage.

A bonus benefit to the diode mod is that if someone forgetfully plugs the transmitter's wall-wart charger into it, the diode will block any current flow back into the lithium battery; the battery will therefore not be overcharged by the (NiMH/NiCd) charger, and you avoid the risk of a lithium fire inside your transmitter.

If you use the diode, make sure to allow for some way to remove it when you need to charge the lithium battery! I did this by making up a short little wire harness that goes from the transmitter board to a 4-pin Deans Micro male plug; this harness contains the diode in the positive lead.

The 4-pin Deans Micro plug mates with a corresponding female plug, which in turn is wired to my lithium-ion battery. I can pull the two Deans Micro connectors apart, and then charge my battery with one of my lithium-safe battery chargers.

Quote:

Originally Posted by Miami Mike

Forget LiPo's, NiMh cells are the way to go for several good reasons.

My opinion is the exact opposite: forget NiMH, lithium cells are the way to go for several reasons.

Why do I use a lithium battery in my Tx? Because it offers four times the capacity of the stock battery, cost me less than a replacement battery, weighs a lot less than an 8-cell NiMH or NiCd battery, and has negligible self-discharge. I can charge it today, and if I am grounded by rain or work for two weeks, I know it will still be fully charged and ready when I finally get a free minute to go flying.

Several of the folks I fly with get essentially free lithium batteries for their transmitters - they just use their oldest, most worn-out lipo packs, the ones that have so many cycles on them that they will no longer do a good job of powering a model airplane. Those batteries would otherwise end up in the recycling bin - why not use them in your Tx and get the obvious beneifts of long life and negligible self-discharge?

Remember that we're talking about an XP8103 here. It balances nicely from the neck strap whether it has eight NiCd or NiMh cells inside. With Lithium cells, it would probably be top-heavy.

More than once, I've accidentally left one of my XP8103 radios on all night after using it to center the servos on a new plane and then to program it. Luckily, the radios have NiMh cells, so the batteries were still fine after a recharge. Lithium cells would have been totally destroyed.

I still use the original wall chargers that came with my radios. Since the radios normally never come close to being fully drained, I just plug them in at night and they're fully charged by the next morning. A radio converted to Lithium cells can't be recharged with the supplied charger, and might even be dangerous if someone was to accidentally try.

Although radios that have been converted to Lithium cells probably work just fine and stay on frequency just as well, it could still be regarded as an FCC violation to run them on a higher voltage than what was used by the manufacturer to obtain official FCC certification.

Virtually every radio has some sort of safety indication to warn you if the battery is dangerously low, based on voltage. When you convert to NiMh cells, the safety indication is still useful. When you convert to lithium cells, it can no longer be relied upon to give you meaningful and useful information about the state of your battery pack.

Two four-packs of brand new 2500 MAh Energizer AA cells cost less than $20, and if you can solder, it's very easy to build a new pack using the plug and harness from your old NiCd pack.

Remember that we're talking about an XP8103 here. It balances nicely from the neck strap whether it has eight NiCd or NiMh cells inside. With Lithium cells, it would probably be top-heavy.

Okay, I'll go with you on this one - I don't have an 8103.

Quote:

Originally Posted by Miami Mike

More than once, I've accidentally left one of my XP8103 radios on all night after using it to center the servos on a new plane and then to program it. Luckily, the radios have NiMh cells, so the batteries were still fine after a recharge. Lithium cells would have been totally destroyed.

The beeping from the low-voltage warning didn't keep you awake? Perhaps you keep your RC stuff far from your bedroom.

Certainly deep-discharging lithium cells is a bad idea. However I make it a habit to turn my Tx off immediately after use (I don't want to accidentally shoot down some other RC flyer flying his "backyard plane" within a couple of miles of me). So far I have never forgotten to do this.

Quote:

Originally Posted by Miami Mike

A radio converted to Lithium cells can't be recharged with the supplied charger, and might even be dangerous if someone was to accidentally try.

Yes to the first sentence; no to the second, if you use the diode in series with one battery lead that I recommended. The diode prevents any charge current getting into the battery even if someone thoughtlessly hooked up the stock Tx charger.

Quote:

Originally Posted by Miami Mike

Although radios that have been converted to Lithium cells probably work just fine and stay on frequency just as well, it could still be regarded as an FCC violation to run them on a higher voltage than what was used by the manufacturer to obtain official FCC certification.

Use the diode mod, and you will not be running them at higher voltage than stock. Use two diodes in series if truly paranoid.

Quote:

Originally Posted by Miami Mike

Virtually every radio has some sort of safety indication to warn you if the battery is dangerously low, based on voltage. When you convert to NiMh cells, the safety indication is still useful. When you convert to lithium cells, it can no longer be relied upon to give you meaningful and useful information about the state of your battery pack.

Not true here - my DX6 starts beeping at 9 volts at the Tx; with one diode in the battery lead, that corresponds to about 9.7 volts at the actual battery terminals. That's quite a good cutoff voltage for a 3S lithium pack.

Since my Tx has a voltage readout, I usually charge the battery before it gets that low. With two to four weeks of flying time per charge, I usually have plenty of warning when a recharge is needed.

Quote:

Originally Posted by Miami Mike

Two four-packs of brand new 2500 MAh Energizer AA cells cost less than $20, and if you can solder, it's very easy to build a new pack using the plug and harness from your old NiCd pack.

Six 1200 mAh lithium-ion cells at $2 each cost $12 (from All Electronics). I soldered up my pack, just as you did with your NiMH pack. You need special solder, and good soldering skills, as you should NOT overheat lithium cells during soldering.

How about we agree you have your good reasons for preferring NiMH, and I have my good reasons for preferring to go with lithium.